1. Field of the Invention
The present invention relates to a seal structure suited for a variable resistor, especially a small-sized variable resistor for use in measuring instruments, communication equipment, sensors, and other industrial apparatuses or equipment.
2. Description of the Related Art
In recent years, there has been the general tendency that apparatuses or equipment is generally miniaturized and small-sized. Regarding the variable resistors which are internal parts of the apparatuses or equipment, the miniaturization has been advanced. In some cases, the apparatuses or equipment is used in a high moisture environment. Accordingly, for the variable resistors, a water-proof, moisture-proof structure is required.
Generally, a variable resistor comprises a case having a concavity, a resistor substrate having a resistor formed on the surface thereof and disposed on the bottom of the concavity of the case,.and a rotor having a slider fixed to the underside thereof and received rotatably in the concavity of the case. The resistance can be changed by sliding the slider on the resistor of the resistor substrate. The above-mentioned rotor is rotatable with respect to the case. Accordingly, sealing between the rotor and the case is needed. For this purpose, a peripheral groove is formed in the outer periphery of the rotor, and an O ring is disposed into the peripheral groove to perform the seal between the rotor and the case.
However, it is a troublesome work to fit the O ring into the peripheral groove of the rotor. Especially, it is very difficult to fit an O ring into the peripheral groove of a small-sized rotor accurately with the O ring being stretched to be enlarged. Fitting defects readily occur, causing imperfect sealing.
As disclosed in Japanese Unexamined Patent Application Publication No. 10-149907, a variable resistor is known, in which a slider is fixed to the bottom of the concavity of a case, a resistor substrate is attached to the underside of a rotor with the resistor of the substrate facing downward, and the slider is slidable in contact with the resistor. In this variable resistor, to render water-proof and moisture-proof properties, an O ring is placed between the upper side of the rotor and the upper side of the case, and by pressing a cover against the O ring, sealing between the rotor and the case is attained.
In this instance, it is unnecessary to fit the O ring onto the rotor with the O ring being stretched to be enlarged as in the above-described first conventional example. The assemblage workability is advantageously enhanced.
However, in the above-described second conventional example, the rotor is in the rising state during assemblage, due to the spring force of the slider, and therefore, the outer peripheral surface of the O ring is not guided by the case, that is, the O ring is simply placed on the outer periphery in the upper side of the rotor. Thus, the O ring is positioned in the unstable state. In this state, the positional slip or release of the O ring readily occurs if a transportation-vibration or the like is applied at assemblage. If the assemblage is carried out in the state that the O ring is positionally slipped, the O ring will be incorrectly engaged there, causing the imperfect sealing.
Accordingly, it is an object of the present invention to provide a variable resistor in which an O ring can be incorporated simply and stably, and imperfect seal is prevented.
It is another object of the present invention to provide a variable resistor of which the number of parts is not increased, and the miniaturization can be realized, due to the O ring of which the release is prevented by utilization of the existing parts.
To achieve the above-described objects, according to the present invention, there is provided a variable resistor which includes a case having a reception space, a slider fixed in the bottom of the reception space of the case, a rotor rotatably received in the reception space of the case, a resistor substrate having a resistor formed on the underside thereof and slidable in contact with the slider, and attached to the underside of the rotor, and an O ring performing the seal between the inner peripheral surface of the reception space of the case and the outer peripheral surface of the rotor, the rotor having a step portion formed in the outer peripheral surface on the underside thereof with the lower-side portion of the step portion having a short diameter, the O ring being fitted onto the short diameter portion of the step portion, the whole or a part of the resistor substrate being formed so as to have a diameter longer than that of the short diameter portion of the step portion of the rotor, and the resistor substrate being fixed to the underside of the rotor whereby the O ring is held between the step portion of the rotor and the resistor substrate.
According to the present invention, there is provided a variable resistor which includes a case-having a reception space, a resistor substrate having a resistor on the upper side thereof and disposed in the bottom of the reception space of the case, a rotor rotatably received in the reception space of the case, a slider slidable in contact with the resistor and attached to the underside of the rotor, and an O ring performing the seal between the inner peripheral surface of the reception space of the case and the outer peripheral surface of the rotor, the rotor having a step portion formed in the outer peripheral surface on the underside thereof with the lower-side portion of the step portion having a short diameter, the O ring being fitted onto the short diameter portion of the step portion, the whole or a part of the slider being provided with a base sheet portion having a diameter longer than that of the short diameter portion of the step portion of the rotor, the base sheet portion of the slider being fixed to the underside of the rotor, whereby the O ring is held between the step portion of the rotor and the base sheet portion of the slider.
In the variable resistor according to the present invention, for assemblage, first, the O ring is fitted onto the short diameter portion of the step portion of the rotor, and the resistor substrate is fixed to the underside of the rotor, whereby the O ring is prevented from being released in the axial direction. In this state, the rotor having the resistor substrate fixed thereto is inserted into the reception space of the case. Since the slider is fixed in the bottom of the reception space of the case, the slider and the resistor substrate are contacted together under pressure so that the rotor gets into the rising state. However, since the O ring is prevented from being released, due:to the step portion of the rotor and the resistor substrate, no positional sipping or releasing occurs. The O ring is prevented from being incorrectly engaged or being detached. Thus, the sealing performance is secured.
Further, for incorporation of the O ring, it is needed simply to insert the O ring onto the short diameter portion of the rotor and attach the resistor substrate to the underside of the rotor. Accordingly, it is unnecessary to stretch the O ring for enlargement. The attachment can be performed simply and stably, even if the rotor and the O ring are small-sized parts. Further, the resistor substrate prevents the O ring from being released. Thus, an especial part for preventing the release of the O ring is unnecessary, and the number of the parts is not increased. In addition, since the resistor substrate is attached directly to the underside of the rotor, the height of the rotor can be reduced, which serves to miniaturize the variable resistor.
As a method of attaching the resistor substrate to the underside of the rotor, adhesion, heat caulking, screwing, and so forth may be used. Preferably, a protuberance is formed in the underside of the rotor, an engaging portion into which the protuberance is fitted under pressure is formed in the resistor substrate, and the protuberance of the rotor is fitted under pressure into the engaging portion of the resistor substrate, whereby the rotor and the resistor substrate are joined together integrally, rotatably. By this, the assemblage can be easily performed as compared with other fixing methods. In addition, desirably, eliminated are conduction defects which may be caused by an adhesive or the like adhering to the resistor and an electrode. After the rotor is incorporated into the case, the resistor substrate is pressed by the spring force of the slider so as to be further fitted onto the rotor. Accordingly, there is no possibility that the resistor substrate is released from the rotor while the variable resistor is used.
As regards the method of incorporating the rotor into the case, it is not restrictive that the slider is fixed to the bottom of the case beforehand, and the rotor is incorporated from the upper side. Preferably, a restraining wall for restraining the rotor from being released upward is formed on the upper end of the case, the rotor is inserted through an open portion provided in the bottom of the case, and a slider block having the slider fixed thereto is fitted into the open portion in the bottom of the case. In this instance, preferably, the gap between the open portion in the bottom of the case and the slider block is sealed with a resin to perform both of the fixing and the sealing between the open portion and the slider block.
According to the present invention, the arrangement of the slider and the resistor substrate is reversed as compared with that according to the first aspect of the present invention. That is, the resistor substrate is provided in the bottom of the case, and the slider is attached to the underside of the rotor. The resistor substrate may be formed separately from or integrally with the case.
In this instance, the O ring is prevented from being released by forming in the slider a base sheet portion of which the diameter is wholly or partially longer than that of the short diameter portion of the step portion of the rotor, and fixing the base sheet portion to the underside of the rotor. Accordingly, the O ring can be securely attached without the positional slipping or departing of the O ring occurring. Also in this instance, for the attachment, the O ring is simply inserted onto the short diameter portion of the step, portion of the rotor. Thus, the incorporation work can be easily performed, even if the rotor and the O ring are small-sized parts.
Preferably, a protuberance is formed in the underside of the rotor, and fitted under pressure into an engaging portion formed in the base sheet portion of the slider, whereby the rotor and the base sheet portion of the slider are joined together. This assemblage can be easily performed as compared with other fixing methods.
Also preferably, the rotor is incorporated into the case from the lower side thereof, the open portion in the bottom of the case is closed with the resistor substrate, and moreover, is sealed with a resin.
Preferably, the upper side of the case is provided with an open portion to which a part of the rotor is exposed, and in the open portion of the case, a cover for restraining the rotor from rising and holding the rotor rotatably is attached. In this instance, the rotor can be incorporated into the case form the upper side thereof. If the case is formed so as to be bottomed, it is unnecessary to seal the bottom of the case with a resin.